readme/build: fix gyp clone url.

Cloning gyp via https://git.chromium.org/external/gyp.git
throws a connection failed error. Switching to:
  `git clone https://chromium.googlesource.com/external/gyp.git`
   OR
  `svn co http://gyp.googlecode.com/svn/trunk build/gyp`
works.
This addresses issue #1547